Awe, they worry us, I know...Him darkening himself while he's in his tank and under his lights is completely normal, he's doing this on purpose in order to absorb more light and heat. And the markings on his belly are just that, they are just markings. They are often referred to as "stress" markings, but they really aren't, they are just markings like the ones all over their bodies, and they darken with the rest of him when he's trying to attract more light and heat.
Make sure that you check all 3 temperature zones inside his tank to be sure they are within the correct ranges, if they are too low they will stay very dark to try to attract as much heat as possible...Do you have just that stick-on thermometer, or do you also have a Temperature Gun or a Digital Probe Thermometer too? If not, you really need to spend $10 on a Digital Probe Thermometer, because without one (or a Gun) you have no way of measuring his Basking Spot Surface Temperature at all, and this needs to always be between 105-110 degrees for him to be able to properly digest his food, and you don't want it over 110 degrees, as this can dehydrate him and actually make him very sick. The stick-on thermometers can only measure the Ambient or air temperatures on the Hot Side and the Cool Side of his tank, but not the Basking Spot Surface Temp...and they are also usually very inaccurate, off by between 10-20 degrees. So hopefully you already have a Gun or a Digital Probe Thermometer, but if not please get one ASAP, and then measure his Basking Spot Surface Temp with it ASAP.
Also, if you already have or if you buy a Digital Probe Thermometer, be sure that you allow the Probe to sit on the spot you are measuring for at least 20-30 minutes before you read the temperature, as the Probe needs to heat the entire way up to the Temperature...
